Chassis spring finger tortuous path with improved manufacturability

ABSTRACT

Disclosed is a chassis assembly that reduces the amount of electromagnetic radiation entering or leaving the interior of the chassis assembly. The chassis assembly comprises a chassis cover and chassis back. The chassis cover comprises a plurality of dimpled spring contacts and hemmed edges and hemmed lips. The chassis back comprises a chassis back top portion adapted to mate with the chassis cover, such that the chassis back top portion is in good contact with the one or more dimpled spring contacts, providing good electrical conductivity. Further, the chassis assembly is designed such that the mating between the chassis cover and chassis back provides a tortuous path to any electromagnetic radiation disposed to enter or leave the chassis assembly.

FIELD OF THE INVENTION

[0001] The invention is related to manufacturing enclosure assembliesfor reducing electromagnetic interference. More particularly, theinvention is related to a system and method for reducing electromagneticinterference in enclosure assemblies through the use of spring fingersand a tortuous path which the electromagnetic wave must follow to leaveor enter the enclosure assembly.

BACKGROUND OF THE INVENTION

[0002] Electro-magnetic compatibility (EMC) is defined as “the abilityof a device, equipment or system to function satisfactorily in itselectromagnetic environment without introducing intolerableelectromagnetic disturbances to anything in that environment” (IEEEC63.12-1987). As has been shown through many anecdotal incidents, EMChas not always been achieved. Examples include the interference betweena notebook PC and testing equipment, a printer and a desktop PC, or acellular telephone and medical devices. These and other pairs ofelectronic devices often experience electromagnetic interference (EMI).

[0003] The operation of many electrical and electronic devices involvesthe changing of voltage or current levels either intermittently orcontinuously, sometimes at fast rates. This results in the developmentof electromagnetic energy at discrete frequencies and across band offrequencies. The offending circuit radiates this energy into thesurrounding environment. EMI, therefore, can be defined as anyelectromagnetic disturbance that interrupts, obstructs or otherwisedegrades or limits the effective performance of electronics and/orelectrical equipment. If can be induced intentionally, as in electronicwarfare, or, more commonly, unintentionally, as a result of spuriousemissions and responses, intermodulation products and the like. EMC iswhat the engineer attempts to, design into a device, as a result of EMI.

[0004] EMI has two paths to follow when leaving or entering anelectronic circuit: a radiated path and a conducted path. The radiatedsignal will leak out of (or into) gaps, slots, opening and otherdiscontinuities that may be present in the housing structure (enclosureor shield). Conducted signals travel over power, signal and controllines leaving the housing where they are free to radiate in open space,causing interference. Conversely, conducted signals can travel into thehousing in the same manner.

[0005] Shielding effectiveness (SE) is used to evaluate the suitabilityof a shield. Expressed in terms of decibels (wherein the field reductionis expressed as${{- 20}{\log \left( \frac{{FS}_{2}}{{FS}_{1}} \right)}{dB}};$

[0006] where “FS” refers to “Field Strength, in volts-per-meterssquared), the formulae is:

SE _(db) =A+R+B

[0007] where:

[0008] A=Absorption loss (dB);

[0009] R=Reflection loss (dB); and

[0010] B=Correction factor (dB) (for multiple reflections in thinshields).

[0011] For example, a basic shield might reduce the emergent field toone-tenth of the initial strength, i.e., an SE of 20 dB. A demandingapplication might require a reduction to one-hundred thousandth of theinitial field strength, or shielding effectiveness of 100 dB.

[0012] Absorption loss is the amount of energy strength dissipated asthe wave travels through the shield. The formula for absorption loss isstated as:

A _(db)=1.314(f·σ·μ)^(1/2) ·t

[0013] where:

[0014] f=frequency (MHz);

[0015] μ=permeability relative to cooper;

[0016] σ=conductivity relative to copper; and

[0017] t=shield thickness in centimeters.

[0018] Reflection loss (near-field) depends on the nature of the sourceof the wave and the distance from that source. For a rod, or straightwire antenna, the wave impedance is higher near the source and falls asdistance from the source is increased, but levels out at the plane waveimpedance (377 Ω).

[0019] In contrast, if the source is a small wire loop, the field ispredominately magnetic, and the wave impedance is low near the source.The impedance increases with distance away from the source, but levelsout at 377 Ω at distances beyond approximately one-sixth of awavelength.

[0020] Reflection loss varies according to the ratio of wave-to-shieldimpedance, so it will depend not only on the type of wave, but also onhow far the shield is from the source. An example would be small,shielded equipment.

[0021] Reflection loss in the near field can be calculated as:

R(Electric)db=321.8−(20·log r)−(30·log f)−(10·log(μ/σ)); and

R(Magnetic)db=14.6+(20·log r)=(10·log f)=(10 log (μ/σ))

[0022] where:

[0023] r=distance from source to shield;

[0024] f=frequency (MHz);

[0025] μ=permeability relative to copper; and

[0026] σ=conductivity relative to copper.

[0027] The final component of the SE equation is the calculation of thecorrection factor, B. The formula is:

B _(db)=20 log 10(−exp(2t/σ)).

[0028] This is normally calculated only for magnetic near-fieldconditions and only if the absorption loss is less than 10 dB.Re-reflection within the barrier, due to the absence of significantabsorption, results in increased energy passing through the second faceof the barrier. Thus the correction factor is a negative, indicating areduction in shielding effectiveness.

[0029] For higher frequency electric fields, good shieldingeffectiveness can usually be achieved by the use of thin metal shieldingas the case material or lining, but the assumption is that the shield iscontinuous and fully surrounds the sensitive items without gaps orapertures (a Faraday cage).

[0030] The difficulty in designing a case structure is that openings areunavoidable. Seams needed for manufacturing, access panels and doors,ventilation openings, windows for outside monitoring, and panel mountedcomponents all penetrate the shielded housing, which effectively lowersthe shielding performance of the case. Although slots and gaps areunavoidable, paying attention to the slot length as it relates to thewavelength of the operating frequency of the circuit can be beneficialin shielding design.

[0031] At a gap length of one-half the radiating wavelength (i.e., thecut-off frequency), the RF wave starts to attenuate at a rate of 20 dBper decade ({fraction (1/10)}^(th) of the cut-off frequency) or 6 dB peroctave (½ of cut-off frequency). The highest frequency of RF emission isusually the most critical because it has the smallest wavelength. Whenconsidering the highest frequency, it is important to take into accountany harmonics that may be present; generally only the first and secondharmonics are considered.

[0032] Once the frequency at which an enclosure is radiating RF energyis known, and at what level, it is possible to calculate the maximumpermitted gap, slot or hole in an enclosure. For example, if 26 dB ofattenuation is required at 1 GHz and the wavelength at 1 GHz is 300 mm,the gap or slot will start to attenuate at 150 mm. Therefore, at 1 GHz,the gap should be no larger than: 15 mm ({fraction (1/10)}th) for 20 dBof attenuation, and 3.75 mm (½) for 32 dB of attenuation. A suitableconductive gasket can be used to achieve this level of attenuation bylimiting the gap to a required minimum size. The table below (Table I)lists operating frequencies and corresponding slot or gap sizes for 6dB, 20 dB, or 26 dB attenuation. TABLE I Wave- Wave- Frequency lengthlength −6 −20 −26 (f) (I) C/O (1/21) dB Slot dB Slot dB Slot 50 MHz 6.00m 3.00 m 1.5 m 300 150 mm mm 100 MHz 3.00 m 1.50 m .075 m 150 750 mm mm200 MHz 1.50 m 0.75 m 37.5 m 75 37 mm mm 400 MHz 0.75 m 37 cm 18.75 cm37 19 mm mm 600 MHz 0.5. m 25 cm 12.5 cm 25 12.5 mm mm 800 MHz 37.5 cm19 cm 9.5 cm 19 9.5 mm mm 1 GHz 30.0 cm 15 cm 7.5 cm 15 7.5 mm mm 2 GHz15.0 cm 7.5 cm 3.75 cm 7.5 3.7 mm mm 3 GHz 10.0 cm 5.0 cm 2.5 cm 5.0 2.5mm mm

[0033] Welding, brazing or soldering are the obvious choices for jointsbetween sheets that are permanently secured. The metal faces to bejoined must be clean to promote complete filing of the joint withconductive metal. Screws or rivets are less satisfactory methods tosecure the joints, because permanent low-impedance contact along thejoints between the fasteners is difficult to maintain.

[0034] Conductive EMI gaskets can also be used to reduce EMI. Thefunction of the conductive gasket is to reduce any slots, holes or gapsalong seams and mating surfaces so that RF energy cannot be radiated.EMI gaskets are a conductive medium to fill apertures in the case andprovide a continuous, low-impedance joint. Generally, EMI gaskets aredesigned to provide a flexible connection between two electricalconductors, enabling currents in each conductor to pass through to theother.

[0035] An EMI gasket is selected according to a number of performancecriteria, namely:

[0036] Shielding effectiveness over the specified frequency range;

[0037] Mounting methods and closure forces;

[0038] Galvanic compatibility with the housing structure and corrosionresistance to the outside environment;

[0039] Operating temperature range; and

[0040] Cost.

[0041] Most commercially available gaskets provide enough shieldingperformance to enable a device to meet EMC standards. The key is todesign the gasket into the case or housing properly.

[0042] An important factor is compression, which yields a highconductivity level between gasket and flanges. Poor conductivity betweenthe opposing flanges through the gasket will result in lower shieldingeffectiveness. A total lack of contact along any part of the jointresults in a thin gap capable of acting as a slot antenna. Such anantenna transmits energy at wavelengths shorter than about 4 times thegap length.

[0043] The first step towards ensuring conductivity is to make sure theflange faces are smooth, clean, and treated as necessary to provideconductive surfaces. These surfaces must be masked prior to painting.Then, it is essential that the shielding gasket material is continuouslywell-bonded to the appropriate flange. The compressibility of theconductive gasket is intended to compensate for any flange irregularity.All gaskets have a minimum contact resistance needed to workeffectively. The designer can lower the contact resistance of manygaskets by increasing the compression of the gasket. This, of course,increases the closure force and raises the chances of bowing in thecase. Most gaskets work effectively with 30% to 70% compression of theirfree standing heights. Thus, within the recommended minimum contact,pressure between the two facing low spots is nevertheless enough toensure the adequate conductivity between the gasket and flanges.

[0044] On the other hand, gasket compression should not have to be sohigh that it induces unnatural compression set, which can lead to gasketcontact failure and possible electromagnetic leakage. Flange separationrequirements are essential to control gasket compression to the rangerecommended by the gasket manufacturer. Included in that design is theneed to make sure the flanges are sufficiently rigid so as not to bowsignificantly between flange fasteners. In some cases, additionalfasteners may be needed to prevent bowing in the case structure.

[0045] Compression set also is an important characteristic for jointsthat may be cycled, such as doors or access panels. If a gasket is proneto take a compression set, then the shielding performance will decreasewith each cycle of the door panel. The gasket will require highercompression forces to achieve the shielding levels equivalent to a newgasket. In most applications, this is not possible, and a long-lastingEMI solution is needed. If the case or flange is plastic with aconductive coating, the addition of an EMI gasket should not pose toomany problems. However, the designer must consider the abrasion thatmany gaskets will impart on a conductive surface. Metal gaskets ingeneral tend to be more abrasive on the coated surface. This will reducethe shielding effectiveness of the gasket joint over time and could poseproblems in the future.

[0046] If the case or flange structure is metal, a gasket can be addedby masking off flange surfaces before finishing materials are applied.The use of a conductive mask and peel tape works well if the tape isused on both sides of a mating flange, the EMI gasket can be attached bymechanical fasteners, such as a C-fold™ gasket with integral plasticrivets, or pressure-sensitive adhesive (PSA). The gasket is mounted toone side of the flange, which completes the EMI shielded joint.

[0047] A wide array of shielding and gasket products is available. Theseproducts include beryllium-copper fingers, wire mesh with and without anelastomeric core, expanded metal and oriented wire embedded in anelastomer, conductive elastomers, and metabolized fabric-clad urethanefoam gaskets. Most shielding manufacturers supply estimates of shieldingeffectiveness that can be achieved with the various gaskets. SE is arelative function that depends on aperture, the size of gasket,compression of the gasket and material composition.

[0048] Of all the gasket types, fabric-clad-foam gaskets are the newestand some of the most versatile products on the market. These gaskets canbe formed in a variety of shapes and thickness from 0.5 mm on up, andcan be produced to meet UL flame ratings as well as environmentalsealing standards. A new type of gasket, an environmental/EMI hybrid,can eliminate the need for separate single-purpose seals therebyreducing the cost and complexity of the designer's enclosure. Thesegaskets combine an outer UV-stabilized outer cladding that resistsmoisture, wind and cleaning agents, with a metabolized, highlyconductive interior cladding. Another recent innovation, EMI gasketswith an integral plastic clip, is an attractive alternative totraditional stamped metal gaskets and provides savings in weight,assembly time and cost.

[0049]FIGS. 1A and 1B illustrate a prior art electromagneticinterference (EMI) reducing chassis assembly utilizing clamping, and aclamping and gasket design. FIG. 1A illustrates a simple EMI reducingchassis assembly. In FIG. 1A, chassis cover 2 is mated with chassisbacking 6 utilizing a nut and bolt assembly 4. The material on thechassis that meets with the material on the chassis backing must beunpainted in order to minimize leakage of electromagnetic waves in to,or out of, the chassis assembly. However, in most circumstances,manufacturers prefer that their chassis assemblies, especially on theexterior, be painted, in order for the product to be more attractive.This may be less true for chassis backings, but, in most circumstances,painted surfaces cannot be avoided. In that case, manufacturingdifficulties will ensue because the mating surfaces between the chassiscover and the chassis backing must be free of paint order for goodconduction to occur. When good conduction exists, effectiveelectromagnetic interference reduction properties can be realized. Notethat nut and bolt assembly 4 are but one means for clamping the chassiscover 2 and chassis backing 6 together. Other methods of clamping thancan be used include a simple pressure contact (such that the chassisbacking 6 slides into the chassis cover 2), welding or soldering the twochassis components together.

[0050] An improvement over the EMI reducing chassis assembly design ofFIG. 1A is shown in FIG. 1B in which the same configuration of FIG. 1Ais shown except that an EMI reducing gasket material 8 is placed inbetween the mating surfaces of chassis cover 2 and chassis backing 6.However, in this circumstance, soldering, brazing or welding generallyis not acceptable and so some type of nut and bolt assembly 4 must beutilized. However, as discussed above, care must be taken that the nutand bolt assembly do not either squeeze the gasket material too tight,in which case deformation might ensue, causing electromagnetic radiationto leak in or out of the chassis; or, the nut and bolt might not betightened properly, allowing radiation to be leaked into or out of thechassis, causing the electromagnetic interference.

[0051]FIGS. 2A and 2B illustrate another known EMI reducing chassisassembly utilizing a spring finger design, which is an improvement overthe designs of FIGS. 1A and 1B. FIGS. 2A and 2B illustrate a knownspring finger chassis assembly design for reducing electromagneticradiation into and out of the electronic products contained within thechassis assembly. In FIG. 2A, chassis cover 2 is mated with chassisbacking 6 via spring fingers 10, as shown in FIG. 2B. In FIG. 2B, aplurality of spring fingers 10 are placed all along the exterior of thetop of chassis backing 6. Conversely the spring fingers 10 can also beplaced on the mating surface of chassis cover 2. In either event, thespring fingers are compressed when the chassis backing 6 is placed incontact with chassis cover 2. This causes a tight fit between the twocomponents, discouraging interference-causing-radiation from leaking outof, or admitted into the electronic products within the chassisassembly. While this known design is effective and can be easier andless costly to manufacturer than the prior art design shown in FIG. 1Aor 1B, difficulty arises with multiple removals of chassis backing 6. Insome circumstances, spring fingers 10 can become easily bent and thensubsequently broken, causing large gaps between the mating covers andconsequential EMI to be leaked into and out of the electronic productlocated within the chassis assembly. Replacing bent or broken springfingers 10 on the chassis backing 6 or chassis cover 2 can significantlyincrease the cost of manufacturing.

[0052]FIGS. 3A through 3C illustrate a further known design for reducingthe generation of, or susceptibility to, electromagnetic interference.In FIG. 3A, chassis cover 2 and chassis backing 6 are configured similarto the configuration illustrated in FIG. 1A, except that chassis cover 2is placed underneath the top part of chassis backing 6. Nut and boltassembly 4 (or other means of securing chassis cover 2 and chassisbacking 6) is then used to join the two together. This configuration isknown as a “simple tortuous path”, which is an improvement over theconfiguration illustrated in either FIG. 1A or FIG. 2A. In the simpletortuous path configuration illustrated in FIG. 3C (which is an expandedview of FIG. 3A), electromagnetic radiation that travels in to, or outof, the interior of the chassis has to make at least two turns (firstturn 12 and second turn 14). In FIG. 3B, additional electromagneticinterference protection is afforded by gasket material 8. Again, anyelectromagnetic interference would have to travel through at least twoturns and through the gasket material 8.

[0053] Significant problems ensue from the configurations illustratedand discussed above in reference to FIGS. 1, 2, and 3. One such problemis ease of manufacturing. If simple nut and bolt assemblies are used tojoin the components, then expensive machinery must be fabricated toperform this, or it must be done manually, which also can be expensive.Including gasket material 8 further complicates both attempts atautomation and manual installation. Applying the correct amount oftorque to the nut and bolt assembly, or applying the right amount ofsolder or weld (with or without the gasket) further complicates theprocess. Additionally, none of these methods assures minimizingelectromagnetic radiation from the chassis assembly at a low cost, whichis desired in the manufacturing environment.

[0054] The amount of electromagnetic interference that can be caused byan electronic device is governed by 47 CFR 15, Sections 15.1 through15.525 (47 CFR parts 0-199 describe the role and responsibility of theFederal Communications Commission (FCC)). Such requirements arestringently tested by independent laboratories (e.g., Underwriter'sLaboratory (UL™)), as well as by the manufacturers. If devices are foundnot to be in compliance with 47 CFR 15, penalties and recalls may ensue,both of which are very costly. Thus, it has been shown that there is aneed for a low cost, simple method of manufacturing a chassis assemblyin which electromagnetic interference emitted from the electronicdevices located within the chassis assembly is reduced to the lowestamount possible, yet, cost as little as possible in manufacturing theassembly.

[0055] Thus, it has been shown that a need exists to provide aneffective, less costly and easy-to-manufacture electronics enclosurethat reduces EMI as much as is practically possible. Additionally, aneed exists to do this without imposing additional manufacturing stepsor unnecessary constraints on the final product.

SUMMARY OF THE INVENTION

[0056] An object of the present invention is to substantially solve atleast the above problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, it is an object of thepresent invention to provide a cover and chassis design that providesbetter EMI performance with 12 protected spring fingers along a hemmedfront lip on the cover that seats below a contact rail on the front ofthe chassis, creating not only 12 points of contact, but a tortuous pathto hinder EMI leakage (into or out of the chassis) as well. Because ofthe hemmed front lip, this design also allows the use of pre-paintedsheet metal.

[0057] The purpose of the chassis and cover in accordance with anembodiment of the invention is to provide an enclosure for the printedcircuit board (PCB), provide grounding, and prevent electromagneticradiation from entering or exiting the set top box.

[0058] The objectives of the present invention are substantially met bya chassis assembly for reducing electromagnetic interference from and tothe interior of the chassis assembly, in which the chassis assemblycomprises a chassis cover and a chassis back. The chassis covercomprises a plurality of spring contacts and hemmed lips. The chassisback comprises a portion adapted to mate with the chassis cover, suchthat an interior surface of the chassis back is in secure physicalcontact with one or more of the plurality of spring contacts, andwherein the hemmed lip provides a tortuous path to any electromagneticradiation disposed to enter or leave the chassis assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The novel features believed characteristic of the invention areset forth in the appended claims. The invention itself, however, as wellas other features and advantages thereof will be best understood byreference to the detailed description of the preferred embodiments whichfollows, when read in conjunction with the accompanying drawings, inwhich:

[0060]FIGS. 1A and 1B illustrate a known electromagnetic interference(EMI) reducing chassis assembly utilizing clamping, and a clamping andgasket design;

[0061]FIGS. 2A and 2B illustrates a known EMI reducing chassis assemblyutilizing a spring finger design;

[0062]FIGS. 3A, 3B and 3C illustrate a known EMI reducing chassisassembly utilizing a tortuous path, and a tortuous path and gasketdesign;

[0063]FIG. 4 illustrates a perspective view of a nearly completelyassembled chassis assembly incorporating a tortuous path forelectromagnetic radiation in accordance with an embodiment of theinvention.

[0064]FIGS. 5 and 6 illustrate a top and left side view respectively ofthe chassis cover shown in FIG. 4;

[0065]FIGS. 7A through 7E illustrate an EMI reducing chassis assemblyutilizing a hemmed edge and dimpled spring finger design in accordancewith an embodiment of the invention;

[0066]FIG. 8 illustrates a detail view of an EMI reducing chassisassembly in accordance with an embodiment of the invention demonstratingcontact between the chassis body and the chassis backing; and

[0067]FIG. 9 illustrates a fully assembled section view of the hemmedassembly showing dimpled spring fingers providing electrical conductbetween the chassis backing and chassis cover, and further providing atortuous path for electromagnetic radiation in an accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0068] Several embodiments of the present invention will now bedescribed in detail with reference to the annexed drawings. In thedrawings, the same or similar elements are denoted by the same referencenumerals even though they are depicted in different drawings. In thefollowing description, a detailed description of known functions andconfigurations incorporated herein have been omitted for conciseness.

[0069]FIGS. 4 through 6 illustrate certain views of a chassis assemblywhich incorporates the tortuous path for incoming and outgoingelectromagnetic radiation in accordance with an embodiment of theinvention. Specifically, FIG. 4 illustrates a perspective view of anearly completely assembled chassis assembly. The chassis assembly iscomprised of cover 2 and chassis back 6. The form of this chassisassembly, i.e., a cover slidably interconnected with a chassis back, iswell known and widely used by various manufacturers of electronicequipment. However, what has not been practiced before and what theassembly illustrated in FIG. 4 incorporates, is the tortuous path designof dimpled spring fingers and hemmed edges, which are more fullydescribed in greater detail below. FIGS. 5 and 6 illustrate top and leftside views respectively of the chassis cover 2 shown in FIG. 4.

[0070]FIGS. 7A through 7E illustrate an example of a process for formingthe tortuous path of the chassis assembly shown in FIGS. 4-6 inaccordance with an embodiment of the invention. In FIG. 7A, hemmed edge16 is the point where hemmed lip 22 is bent to form the hemmed assembly.This is fifth bend 26. This is illustrated in FIGS. 7C and 7D. FIG. 7Billustrates the chassis cover interior 26, which is unpainted. This isthe “underside” of chassis cover exterior 20. In a typical manufacturingenvironment, chassis cover 2 is stamped from sheet metal, which can bepainted, but normally is not (because the stamping would then affect thepainted surface). After stamping, the sheet metal forms are then cleanedand painted. Final bends are then made (this includes the sides of thechassis cover 2, and the hemmed edge assembly). Touch-up painting mayfollow, depending on the severity of the bend, and quality of thepainting step. FIG. 7C illustrates a side view of chassis cover 2. Thearrow in FIG. 7C indicates the direction the bend will occur. In FIG.7D, the bend is shown to be approximately three-quarters finished, andhemmed lip 22 is rotating through an arc back under the chassis coverexterior 20. Dimpled spring finger 18 will protrude above the surface ofthe chassis cover exterior 20 when it is completely rotated(approximately 180°) via fifth bend 26 of hemmed edge 16. Thus, fifthbend 26 is at or about 180°.

[0071] Referring to FIG. 7E, chassis cover 2 is bent at a known distancefrom the hemmed edge 16 at or about 90°, as shown in FIGS. 8 and 9. Thisbend forms second bend 34. The distance from the hemmed edge 16 secondbend 34 is formed is determined from the dimensions of the top portionof chassis back 6, as more clearly seen in FIG. 9. As an additional stepin the manufacturing process, a third bend 36 is formed on chassis cover2. This provides the “step” shape as seen in FIG. 7E. The distancesbetween hemmed edge 16 and second bend 34, and between second bend 34and third bend 36 are dependent on the specific chassis design.

[0072] In FIG. 7F, a top view of FIG. 7E, the hemmed lip 22 has beenbent underneath the chassis cover exterior so that the dimpled springfinger 18, which was previously pointing down as illustrated in FIG. 7D,is now pointing up, ready to contact the underside of the top of thechassis backing 6. This assures good electrical conductivity in thechassis assembly. The bends create a tortuous path, which provides theminimization of EMI to, and from, the chassis assembly.

[0073] The chassis assembly illustrated in FIGS. 4 through 9 provideseveral advantages over previous chassis cover assembly designs. First,this design creates a tortuous path. Previous designs have had springfingers on the chassis, with the cover assembling on the top of thechassis spring finger rail. Therefore, if one or more spring fingerswere bent, this allowed a direct line of escape (or entrance) for anelectromagnetic wave (which could cause EMI). Because the dimpled springfingers 18 are now located on the cover, the chassis rail is above thedimpled spring fingers 18 which creates what is called the tortuouspath. For an electromagnetic wave to leak in or out of the settop box,it must now bounce around two bends.

[0074] Referring to FIG. 9, it can be seen that an outgoingelectromagnetic wave 1 must travel through first bend 32, which isformed by the union of hemmed edge 16 and fourth bend 38 (of chassisback 6). First bend 32 is formed to be at or about 90°. Then, theoutgoing electromagnetic wave 1 travels beneath chassis back top portion40 (of chassis back 6) and over the top of chassis cover pre-stepportion 24 (of chassis cover 2) and it encounters second bend 34. Secondbend 34 is formed, during manufacturing, between chassis cover pre-stepportion 24 and chassis step portion 39, to be at or about 90°. Theoutgoing electromagnetic wave 1 (or incoming electromagnetic wave 3) isforced through second bend 34 by the union of chassis back top portion40, chassis cover pre-step portion 24 and chassis cover step portion 39.Any electromagnetic wave that wishes to leave (or enter) the chassisassembly will encounter first bend 32 and second bend 34.

[0075] An additional advantage is provided by the protected dimpledspring fingers 18. In the past, sheet metal spring fingers were easilybent during shipping, handling, and assembly. Since bent spring fingerscan allow electromagnetic interference leakage to and from the settopbox, by not making contact with their mating box, fixtures are often putin place in the manufacturing floor to bend all spring fingers backinginto place prior to assembly. This design, in accordance with anembodiment of the invention, has a protective hemmed edge 16 on thechassis cover 2 which wraps around the dimpled spring fingers 18,protecting them from bending during shipping and handling. A chassisassembly designed in accordance with an embodiment of the invention nolonger requires a fixture in place on the assembly to re-bend the springfingers, thereby reducing manufacturing costs.

[0076] Further, use of this assembly design in accordance with theembodiments of the invention allows the chassis cover 2 to bepre-painted. Because the dimpled spring fingers 18 are bent around in ahem, this allows the under side of the sheet metal on the chassis cover2 to be used as a dimpled spring finger 18 that contacts the chassisback 6. This allows the use of pre-paint or pre-vinyl on the top surface26 of the chassis cover 2, which is a substantial cost savings to a postpaint process with masked spring fingers.

[0077] This design for a settop box is used by Hughes Network System, intheir Helix™ product line, which is a DirecTV™ set top box. Currently, atool is used on the production line on every Gaither™ set top boxchassis to ensure that the spring fingers are in a position such thatcontact will be made with the mating cover. On average, the use of thistool costs the factory $110,000 per year.

[0078] The TiVO™ product, model number HDVR2, utilizes a spring tabdesign on their cover to ensure contact to the chassis. While these tabsare well protected during shipping and handling, the design does notincorporate a tortuous path. If there was a single tab that did not makecontact with the chassis, a potential path for EMI leakage coulddevelop. A chassis assembly designed in accordance with the embodimentsof the invention incorporates not only well protected dimpled springfingers 18, but also a tortuous path making it very difficult for EMI toescape (or enter). It also allows for the use of pre-painted sheetmetal.

[0079]FIG. 8 illustrates a detail view of an EMI reducing chassisassembly designed and build in accordance with an embodiment of theinvention, demonstrating contact between the chassis cover 2 and thechassis back 6. FIG. 9 illustrates a fully assembled section view of thehemmed assembly showing dimpled spring fingers 18 providing electricalconduct between the chassis back 6 and chassis cover 2, and furtherproviding a tortuous path for electromagnetic radiation in accordancewith the embodiments of the invention.

[0080] In FIG. 9, the painted surface is located on top of the chassiscover 2 and the dimpled spring fingers 18 are not painted, because theyare relied upon to be the contact surface with the interior of chassisback 6. Chassis back 6 slides over the hem assembly (comprised of hemmededge 16, hemmed lip 22, dimpled spring fingers 18 and chassis coverpre-step portion 24), the underside of the chassis back top portion 40,and contacts the top of dimpled spring finger 18, thereby producing goodelectrical conductivity between the chassis back 6 and chassis cover 2.As described above, a tortuous path to any electromagnetic waves wishingto enter or escape the chassis assembly is also provided. Anyelectromagnetic radiation disposed to exit the chassis assembly (e.g.,outgoing electromagnetic wave 1), must first pass through first bend 32,and then second bend 34. Conversely, any electromagnetic wave disposedto enter the chassis assembly (e.g., incoming electromagnetic wave 3),must first pass through second bend 34, and then first bend 32.

[0081] As the chassis cover 2 slides over the hem assembly (describedabove) of chassis cover 2, hemmed edge 16 slides under chassis back topportion 40 until they two are fully seated. In one embodiment of theinvention, hemmed lip 22 of the chassis cover 2 includes 12 dimpledspring fingers 18 to create the under side of the hemmed lip 22, though,as one skilled in the art can appreciate, the number of dimpled springfingers 18 is a design choice, and depends on many factors. Because thedimpled spring fingers 18 are folded under the chassis back top portion40 along the hemmed lip 22, they are well protected during shipping andhandling, eliminating factory handling issues. On each of these dimpledspring fingers 18 is a dimple that makes contact to the mating surfaceon the chassis (the underside of chassis back top portion 40) as shownin the fully assembled section view (FIG. 9). This not only guarantees12 points of contact (as one example) between the chassis back 6 andchassis cover 2, but also creates a tortuous path, forcingelectromagnetic waves to make two turns before escaping or entering thechassis assembly. Also, since the dimpled spring fingers 18 are foldedup under the hemmed lip 22, this allows the use of pre-painted sheetmetal because the underside of the sheet metal (which is not painted) isthe contacting surface.

[0082] While various embodiments of the invention has been shown anddescribed with reference to the figures thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thevarious embodiments of invention as defined by the appended claims.

What is claimed is:
 1. A chassis assembly designed to reduceelectromagnetic radiation from entering or leaving the interior of thechassis assembly, comprising; a chassis cover, the chassis covercomprising a hem assembly adapted to mate with a chassis back such thatan interior surface of the chassis back is in secure physical contactwith the chassis cover; and wherein the hem assembly mated with thechassis back provides a tortuous path to any electromagnetic radiationdisposed to enter or leave the chassis assembly.
 2. The chassis assemblyaccording to claim 1, wherein the tortuous path comprises: a first bendof substantially 90 degrees, formed between by the hem assembly and achassis back top portion; and a second bend of substantially 90 degrees,formed by bending the chassis cover between a chassis cover pre-stepportion and chassis cover step portion.
 3. The chassis assemblyaccording to claim 1, wherein the hem assembly comprises: a chassiscover pre-step portion; a hemmed lip, adapted to comprise one or moredimpled spring fingers; a hemmed edge, which is a bending point,allowing the hemmed lip to be bent under the chassis cover pre-stepportion at or about 180°, providing the dimpled spring fingers to engagean underside of the chassis cover.
 4. The chassis assembly according toclaim 1, further comprising: one or more dimpled spring fingers; and achassis back top portion securely contacting the one or more dimpledspring fingers to provide good electrical conductivity between thechassis back and chassis cover.
 5. A chassis cover used in a chassisassembly designed to reduce electromagnetic radiation from entering orleaving the interior of the chassis assembly, comprising: a hemassembly, comprising a chassis cover pre-step portion, a hemmed lip, ahemmed edge and one or more dimpled spring fingers; and a chassis covertop portion, wherein a third bend is made between the chassis cover topportion and the chassis cover step portion, and a second bend is madebetween the chassis cover step portion and the chassis cover pre-stepportion, and the hemmed lip is bent at the hemmed edge between thechassis cover pre-step portion and the hemmed lip to form a fifth bend,such that that one or more dimpled spring fingers located on anunderside of the hemmed lip now point in a direction substantiallyperpendicular to a topside of the chassis cover pre-step portion andchassis cover top portion.
 6. The chassis cover according to claim 5,wherein: the third bend between the chassis cover top portion and thechassis cover step portion is at or about 90°.
 7. The chassis coveraccording to claim 5, wherein: the second bend between the chassis coverstep portion and the chassis cover pre-step portion is at or about 90°.8. The chassis cover according to claim 5, wherein: the fifth bend atthe hemmed edge is at or about 180°.
 9. A method of manufacturing achassis assembly, the chassis assembly designed to reduceelectromagnetic interference, the method comprising: forming a thirdbend between a chassis cover top portion and a chassis cover stepportion; forming a second bend between a chassis cover step portion anda chassis cover pre-step portion; forming a fifth bend at a hemmed edge,wherein the hemmed edge resides between a hemmed lip, the hemmed lipcomprising one or more dimpled spring fingers, and the chassis coverpre-step portion; forming a fourth bend between a chassis back topportion and the chassis back, wherein a formed chassis cover comprisesthe chassis cover top portion, the chassis cover step portion, thechassis cover pre-step portion, the hemmed edge, the hemmed lip and theone or more dimpled spring fingers; and seating the formed chassis coverwith the chassis back to create a tortuous path.
 10. The methodaccording to claim 9, wherein the step of seating the formed chassiscover with the chassis back comprises: sliding the hemmed edge under thechassis back top portion formed from the chassis back, such that the oneor more dimpled spring fingers is in contact with an underside of thechassis back top portion, providing good electrical conductivity. 11.The method according to claim 9, wherein: the second, third and fourthbends are at or about 90°; and the fifth bend is at or about 180°. 12.The method according to claim 10, wherein the tortuous path comprises: apath formed by the second bend, the portion between the underside of thechassis back top portion and a top of the chassis cover pre-stepportion, and a first bend formed by the union of the fourth bend and thehemmed edge.
 13. The method according to claim 12, wherein: the firstbend is at or about 90°.